The present invention relates to a process for the manufacture of dense sintered shaped articles of polycrystalline boron carbide.
Because of the poor sinterability of boron carbide powder, dense sintered shaped articles of boron carbide have to be manufactured industrially by the so-called hot-pressing or pressure-sintering process. In this way, it is possible to manufacture sintered articles of boron carbide having a density of from 2.50 to 2.52 g/cm.sup.3 (the latter figure corresponding to 100% of the theoretical density of boron carbide) and practically free of pores, with a residual porosity of less than 1%. Such particles generally have a flexural strength within the range of from 300 to 400 N/mm.sup.2, and this serves to characterize their mechanical strength.
This process has the disadvantage that it can be used for the manufacture only of relatively small and geometrically simple shaped articles. Articles of more complex shape cannot be made in this manner, but have to be machined from more simple shapes. Because boron carbide is the next hardest known material after diamond and boron nitride, such machining operations have to be carried out using diamond tools and are time-consuming and expensive. Moreover, the continuous operation of the hot-pressing process is unusually expensive.
An alternative process known for the sintering of articles of other materials is the pressureless sintering process, which enables articles of complex shape to be manufactured relatively cheaply without the need for expensive final machining operations. Moreover, it can also be carried out in a continuous manner using pusher or band type furnaces, thus permitting the economical mass production of shaped articles. Many attempts have therefore been made to produce articles of boron carbide by the pressureless-sintering process. It was found, however, that, using sintering temperatures of from 2250.degree. to 2300.degree. C. with sintering time, of 1 to 2 hours, only moderately dense articles of boron carbide could be produced by this method. Such articles had densities of from 2.0 to 2.2 g/cm.sup.3 (corresponding to from 80 to 87% TD - percent of the theoretical density of boron carbide) and flexural strengths of from 50 to 100 N/mm.sup.2. In order to achieve densities above 90% TD, and thus increase the flexural strength to up to 200 N/mm.sup.2 (cf. K Adlassnig, Planseeberichte fur Pulvermetallurgie vol. 6 (1958), pages 92-102), it was necessary to use such high temperatures so that surface melting occurred, which, of course, impaired the dimensional stability of the articles.
More recent investigations have also shown that boron carbide powder having an approximately stoichiometric composition can be densified to a high degree only in the immediately premelt temperature range, namely at about 2300.degree. C. (cf. P.S. Kislyi and B.L. Grabtschuk, Bericht uber Untersuchung der Gesetzmassigkeiten bei Erzeugung des Hochdicten Borcarbids durch Sinterung, 4th European Powder Metallurgy Symposium (Grenoble, France), 1975, INIS-mf-2082; Chem. Abs. 87 (1977) 140 112). At such temperatures, practically poreless boron carbide articles can be produced by pressureless-sintering, with residual porosities of from 0.5 to 1% and grain sizes in the microstructure of at least 10 to 15 .mu.m, but the strength of such articles is from 15 to 20% lower than that of similar articles produced by hot-pressing.
The manufacture of dense sintered boron carbide articles containing beryllium carbide, by molding a mixture of submicron powders of boron carbide and beryllium carbide into a shaped green body, and then subjecting the body to pressureless sintering at temperatures of from 2200.degree. C. to 2280.degree. C. is desribed in U.S. Pat. No. 4,005,235. The articles produced by this process are polycrystalline, with a two-phase microstructure with round boron carbide grains having diameters of from 30 to 300 .mu.m and smaller beryllium carbide grains, and densities of at least 85% TD, but no information is given regarding their mechanical strength. The boron carbide submicron powder used had a specific surface area of 16.1 m.sup.2 /g, corresponding to a particle size of less than 0.1 .mu.m, but it was found that this particle size alone was not of decisive importance, because articles produced from this powder without beryllium carbide, by pressureless sintering at a temperature of up to 2260.degree. C., had densities of up to only about 72% TD.
It is thus apparent that highly densified boron carbide articles can be produced by the pressureless-sintering method only by using very high sintering temperatures of about 2300.degree. C. or by using beryllium carbide conjointly. The former method requires very accurate temperature control in order to avoid the danger of melting the boron carbide, and such control is very difficult in that range. The latter method has the disadvantage that beryllium carbide is highly toxic and very moisture-sensitive and, moreover, the presence of impurities such as beryllium carbide, as well as magnesium and iron, in amounts exceeding about 1% practically precludes the articles from use in the field of nuclear technology, for example as neutron-absorbers.